Concrete 101


Slump is a measure of concrete consistency or fluidity. Consistency is the ability of freshly mixed concrete to flow. For given proportions of cement and aggregates (without admixtures), the higher the slump, the wetter the mix. Four-inch (4”) slump is very common with normal weight concrete and is a good average slump for pumping. Above average slump – due to the addition of water – considerably reduces the strength, durability, and permeability of concrete and can cause segregation.

When possible, admixtures should be used instead of water to achieve higher slump. Slump loss through the pump can negatively affect pumping. Aggregate will absorb large amounts of water when under the pressure of the pump. This can cause a blockage and be completely unpumpable. Such aggregate should be soaked long enough to achieve Saturated Surface Dry (SSD). SSD aggregate cannot absorb any more water.

When pumping at longer distances a higher slump must be used in order for the concrete to flow.